The development of highly absorbent articles for use as disposable diapers, adult incontinence pads and briefs, and catamenial products such as sanitary napkins, are the subject of substantial commercial interest. A highly desired characteristic for such products is thinness. For example, thinner diapers are less bulky to wear, fit better under clothing, and are less noticeable. They are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per diaper unit.
The ability to provide thinner absorbent articles such as diapers has been contingent on the ability to develop relatively thin absorbent cores or structures that can acquire and store large quantities of discharged body fluids, in particular urine. In this regard, the use of certain particulate absorbent polymers often referred to as "hydrogels," "superabsorbents" or "hydrocolloid" materials has been particularly important. See, for example, U.S. Pat. No. 3,699,103 (Harper et al), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harmon), issued Jun. 20, 1972, that disclose the use of such particulate absorbent polymers in absorbent articles. Indeed, the development of thinner diapers has been the direct consequence of thinner absorbent cores that take advantage of the ability of these particulate absorbent polymers to absorb large quantities of discharged aqueous body fluids, typically when used in combination with a fibrous matrix. See, for example, U.S. Pat. No. 4,673,402 (Weisman et al), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al), issued Jun. 19, 1990, that disclose dual-layer core structures comprising a fibrous matrix and particulate absorbent polymers useful in fashioning thin, compact, nonbulky diapers.
These particulate absorbent polymers were previously unsurpassed in their ability to retain large volumes of fluids, such as urine. A representative example of such particulate absorbent polymers are lightly crosslinked polyacrylates. Like many of the other absorbent polymers, these lightly crosslinked polyacrylates comprise a multiplicity of anionic (charged) carboxy groups attached to the polymer backbone. It is these charged carboxy groups that enable the polymer to absorb aqueous body fluids as the result of osmotic forces.
Absorbency based on capillary forces is also important in many absorbent articles, including diapers. Capillary forces are notable in various everyday phenomena, as exemplified by a paper towel soaking up spilled liquids. Capillary absorbents can offer superior performance in terms of the rate of fluid acquisition and wicking, i.e. the ability to move aqueous fluid away from the point of initial contact. Indeed, the dual-layer core absorbent structures noted above use the fibrous matrix as the primary capillary transport vehicle to move the initially acquired aqueous body fluid throughout the absorbent core so that it can be absorbed and retained by the particulate absorbent polymer positioned in layers or zones of the core.
Other absorbent materials capable of providing capillary fluid transport are open-celled polymeric foams. Indeed, certain types of polymeric foams have been used in absorbent articles for the purpose of actually imbibing, wicking and/or retaining aqueous body fluids. See, for example, U.S. Pat. No. 3,563,243 (Lindquist), issued Feb. 6, 1971 (absorbent pad for diapers and the like where the primary absorbent is a hydrophilic polyurethane foam sheet); U.S. Pat. No. 4,554,297 (Dabi), issued Nov. 19, 1985 (body fluid absorbing cellular polymers that can be used in diapers or catamenial products); U.S. Pat. No. 4,740,520 (Garvey et al), issued Apr. 26, 1988 (absorbent composite structure such as diapers, feminine care products and the like that contain sponge absorbents made from certain types of super-wicking, crosslinked polyurethane foams).
The use of absorbent foams in absorbent articles such as diapers can be highly desirable. If made appropriately, open-celled hydrophilic polymeric foams can provide features of capillary fluid acquisition, transport and storage required for use in high performance absorbent cores. Absorbent articles containing such foams can possess desirable wet integrity, can provide suitable fit throughout the entire period the article is worn, and can minimize changes in shape during use (e.g., swelling, bunching). In addition, absorbent articles containing such foam structures can be easier to manufacture on a commercial scale. For example, absorbent diaper cores can simply be stamped out of continuous foam sheets and can be designed to have considerably greater integrity and uniformity than absorbent fibrous webs. Many absorbent cores made from such fibrous webs fall apart during use. Such foams can also be molded in any desired shape, or even formed into integral, unitary diapers.
Particularly suitable absorbent foams for absorbent products such as diapers have been made from High Internal Phase Emulsions (hereafter referred to as "HIPE"). See, for example, U.S. Pat. No. 5,260,345 (DesMarais et al), issued Nov. 9, 1993 and U.S. Pat. No. 5,268,224 (DesMarais et al), issued Dec. 7, 1993. These absorbent HIPE foams provide desirable fluid handling properties, including: (a) relatively good wicking and fluid distribution characteristics to transport the imbibed urine or other body fluid away from the initial impingement zone and into the unused balance of the foam structure to allow for subsequent gushes of fluid to be accommodated; and (b) a relatively high storage capacity with a relatively high fluid capacity under load, i.e. under compressive forces. These HIPE absorbent foams are also sufficiently flexible and soft so as to provide a high degree of comfort to the wearer of the absorbent article, and can be made relatively thin until subsequently wetted by the absorbed body fluid.
The nature and characteristics of the HIPE absorbent foams are very much dependent on the type of components and the process conditions used to form the HIPE. This includes the emulsifier used in preparing the HIPE. HIPEs, and especially HIPEs having very high ratios of water phase to oil phase, tend to be unstable. Very slight variations or changes in, for example, monomer content of the oil phase, temperature conditions, shearing conditions, or the degree of agitation involved can cause such emulsions to "break" or to separate to at least some degree into their distinct water and oil phases. Even if stable HIPE can be realized, emulsion processing and subsequent polymerization steps can cause coalescence of the relatively small water droplets formed in the HIPE. This can lead to inconsistencies in the cell size of the resultant foam, thus making them less useful for absorbing aqueous fluids.
The properties of the emulsifier used in making the HIPE can have other important effects on the fluid handling properties and characteristics of the resultant HIPE absorbent foam. The monomers used in making HIPE absorbent foams normally result in polymers that would be hydrophobic in the absence of a hydrophilizing surfactant. Even after the HIPE foam is washed and dewatered, some of the emulsifier typically remains within and on the surface of the foam. If it has the right surfactant properties, this residual emulsifier can hydrophilize this normally hydrophobic foam so as to make it capable of being wetted by, and thus absorbing, aqueous fluids.
Although there have been many emulsifiers that have been used to prepare stable HIPEs, a number of these prior emulsifier systems have significant drawbacks. Some of the problems of these prior emulsifier systems include: (1) chemical complexity and variability in composition; (2) variability in performance; (3) relatively low levels of desired interfacially active components capable of forming and stabilizing the HIPE against coalescence at preferred droplet sizes, water:oil ratios, emulsifier levels, and processing conditions; and (4) high levels of components that are insoluble or insufficiently soluble in the oil phase used to prepare the HIPE. These problems can necessitate the use of higher levels of the emulsifier system, the need for an extra processing step to separate oil-phase insoluble components (e.g., sludge) from the oil phase prior to formation of the HIPE, and/or extensive purification steps to make the emulsifier more active, make the emulsifier less variable, reduce the level of components with insufficient oil-phase solubility, etc. All of this increases the cost and process complexity in making HIPE absorbent foams.
Accordingly, it would be desirable to be able use an emulsifier system that: (1) provides a sufficiently stable HIPE for making absorbent foams; (2) provides a relatively consistent and uniform water droplet size in the HIPE; (3) is less chemically complex and variable in composition; (4) has higher levels of the desired interfacially active components; and (5) has lower levels of components that are insoluble or insufficiently soluble in the oil phase used to form the HIPE. It also would be desirable if the emulsifier system could hydrophilize the resultant HIPE absorbent foams even after washing and dewatering.